Paper pulp bleaching process



Patented Aug. 23, 1932 UNITED srarss PATEINTQQOFFICFE WILHELM msonxmn.or ANTiOGH, cann'onum, Assmn'on 'rounnarwns'rnnn ELECTED-CHEMICAL G0,,on sen nnanorsco onmronmafa' oonronarrou or CALIFORNIA No Drawing.

This invention relates to the bleaching of wood pulp, either ground woodor mechanical pulp, or chemical wood pulp as derived from the soda,sulphate or sulphite'processes and the object of the process is tolighten thecolor of such pulps while preserving the strength and paperyield to the maximum. The purpose of my process is not to produce highlybleached or white pulp but rather to cause a lightening of the originaldark color.

For instance, ground wood pulp can not be converted into a white pulpbecause all of the lignin and incrusting matters are still presant, andwhich preclude the use of oxidizing agents as for instance chlorine sonecessary for producing white pulp. The dark color of such pulp can belightened somewhat by treatment with reducing agents, and some of thesehave been so used for some time.

It is also common knowledge that sulphur dioxide gas or bisulphiteliquor will brighten 'or lighten dark pulps to some extent. The actionhowever is slow and the quantity or sulphite necessary to use isconsiderable.

I have discovered that the compound, sodium hydrosulphite f the chemicalcomposition NmS O, has a lightening action on the colorof wood pulpinfinitely greater and far more rapid than sulphur dioxide or bisul-'phit-es. I found that sodium hydrosulphite when added to very darklycolored mechanical Wood pulp in a quantity of one quarter of one percent by weight and thoroughly mixed therewith in the presence of water,produced a lightening of the color far beyond any 'efiect obtainablewith the bisulphites, and

that the whole process was completed in very before going to thepaper-machlnes.

Ground wood which has been in storage 40 for some time gradually darkensand profew minutes.

duces a paper, after treatment in the ordinary way, which isconsiderably oil color, whereas a hydrosulphite added to the pulp inquantities of as little as one quarter of one per cent of the weight ofthe dry ground wood pulp will bring back the color of. fresh groundwood, and a very small additional quantity will lighten the color beyondthe original color. I

Kraft pulp as produced by the sulphate rarnnrunr BLEAGHRQ'G rnoonss 1 5Application filed December 3, 192a. -Se'rla1 No. 323,575.

bleached sulphite" solved in water and run as an aqueous soluprocess isnaturally very dark'in color and. can be considerably improved by theuse of one to two per cent of a hydrosulphite.

Even, the color of-sulphite pulp can be considerably lightened andimproved by small quantities of a; hydrosulphitc, without. of coursereaching the color of the so called pulp produced with hyp0chlorites.-:

I have already mentionedthe rapidity of the action of the hydrosulphitein treatment of paper pulp and it is quite remarkable that this actionis hardly infiuencedby-fltemperature ranges between 50 F. and 120 F. sothat at the rather low temperatures of approximately 50? F. the effectisjust the same. 7 Sodium hydrosulphite has beenmentioned above becauseit is theycheapest and best known representative of ,these compounds.But other hydrosuiphites, such as vpotassium or ammonium hydrosulphite,calcium, or. those of the alkaline earth group have been used by mewithsatisfactory results, and zinc. hydrosulphite (2116 0;) which is-particularly easy to manufacture has also been found suitable for thispurpose.

A further advantage of the use of a hydro-l sulphite for the bleachingofpaper pulp .is

the ease of its application. It can be dis- 80 tion into amixergorbeater-whcre the pulp is being mixed or it can be added as asolid direct to the wet pulpfl'and thoroughly incorporated therewith.'Since the reaction is completed within very few minutes it is onlynecessary to add the hydrosulphite to the beater or mixer in which thepulp is agitated As an example of its application to news print paper2,300 pounds, of ground wood pulp were mixed with 400 pounds of sulphitepulp to a stock density of approximately five per cent. Five andthree-quarters pounds of anhydrous sodium hydrosulphite were added tothis mixture. Ten minutes later the necessary sizing and bluingingredients were added to 'thejbatch and the material was run directlyon the paper ma chine. 7

News print paper resulting from a mixture A aspen Re ssue of about 83%ground wood to about 17% bleached sulphite pulp may be closelyapproximated in grade by using unbleached sulphite pulp with the groundwood if a small quantity of a hydrosulphite is incorporated in theproportions indicated. ,The

advantages are in substituting the cheaper unbleached sulphite pulp forthe bleached pulp ordinarily required.

Since various quantities of the chemical from .25 to 10% of the weightof the pulp produce corresponding degrees of action on the color of thepulp and the amount to be used will be governed entirely by the effectsdesired, I do not confine myself to any particular quantity orpercentage of cal to the pulp treated.

I claim: 7

l. The process of lightening the color of mechanical wood pulp whichcomprises agitating the same substantially at room temperature for aboutten minutes with an aqueous solution of a hydrosulphiteas substantiallythe only active agent therein and in amount ranging from 25% to 10% ofthe weight of the pulp without subsequent washing. i

2.-Tl1e process of li htening thecolor of mechanical wood pulp whichcomprises agitating the same substantially at room temperature for aboutten minutes-with an aqueous solution of sodium hydrosulphitesubstantially as the onlyactiveagent therein and in amount ranging from25% to 10% of the weight of the pulp Without subsequent washing.

3. The process of making paper from mechanical :wood pulpywhichcomprises agitating the pulp at substantially room temperature for ashort time with an aqueous solution of a hydrosul hite as substantiallythe only active'bleachmg agent therein and in amount ranging from 25% to10% of the weight of the pulp, and without subsequent washing convertingthe pulp into paper before reversion of color can take place in thepulp. V

' WILHELMLIHRSGHKIND.

the chemi;

